Small openings partly useless to matadors (7)
I believe the answer is:
stomata
'small openings' is the definition.
'stomata' can be an answer for 'openings' (stoma is a kind of opening). I'm not sure about the 'small' bit.
'partly useless to matadors' is the wordplay.
'partly' indicates the answer is hidden within the clue (part of the clue is used).
'STOMATA' can be found hidden inside 'useless to matadors'.
Can you help me to learn more?
(Other definitions for stomata that I've seen before include "Minute leaf pore" , "Openings" , "Plant pores" .)